GB1336876A - Electric current regulators - Google Patents

Abstract

1336876 Automatic current and temperature control E OTT 8 Dec 1970 [2 Jan 1970] 58285/70 Heading G3R The R M S value of current through a load RA, which may comprise the heater of a protective housing associated with the measurement of fluid parameters, is maintained at a desired value by phase-angle control of a Triac T15 whose firing signal is dependent on a first signal representative of the arithmetic average value of the current and a second signal representative of a pre-set signal corresponding to the desired value, the latter signal being varied in dependence on the phase angle in each halfwave for which the current flows. Figure 2 shows the main components of the regulator in simplified form, the details of these and additional components also being described, Fig. 3 (not shown). The arithmetic average value of current is provided by a current transformer m feeding a resistor-diode bridge supplying an actual value signal through a resistor-capacitor network. The pre-set signal is obtained from a rectifier and voltage divider combination A, B whose output is passed, after combination with the modifying signal dependent on the phase angle of the current, to a differential amplifier E, H also receiving the actual value signal. The modifying signal is obtained from a resistorcapacitor-Zener diode circuit in which the capacitor voltage is a function of the phase angle. The amplifier E, H consists of a resistortransistor bridge E supplying an error signal to another differential amplifier circuit H via an RCR network providing a proportionalintegral regulating action. A zero-voltage switch I produces discharge of a capacitor during each zero crossing of the supply voltage, a Schmitt trigger K being triggered when the capacitor voltage reaches a predetermined value and causing a further capacitor to be charged to a predetermined voltage for discharge through an ignition stage L which fires the Triac T15. If it is desired to provide temperature control of the aforementioned protective housing, the temperature of the latter is sensed by a thermistor forming part of a Schmitt trigger M, which controls members I, K to cause current to be supplied when the temperature falls below a predetermined value. Switch I can be blocked by the member M to prevent firing of the Triac when the temperature is at its predetermined value.